The 29th Infantry Division was the first unit to be trained at Camp McClellan. This division was comprised of troops from New Jersey, Virginia, Maryland and District of Columbia. The first troops arrived in late August, 1917, and they and their commander, Major-General Charles G. Morton received a formal welcome to Anniston. Community relations were forged with the election of a special town representative, W. P. Acker, assigned to deal with the military.

By November, all officers and enlisted men of the division, totaling over 27,000 individuals, had arrived. Draftees arrived later. In Morton's own words, it took "a generous stretch of the imagination to see in this beginning the magnificent fighting unit which was to later form". The history of the 29th Division indicates that the training at the camp was hard. A network of trenches, dugouts, and command posts had been built to further the soldier's training. Stories indicate that fire call and temporary moves from one place in the camp to another to prepare the troops for what might be ahead in Europe were legion, usually occurring at night. The 29th Division remained at Camp McClellan until June 1918, when orders arrived sending the troops to France. The troops took heavy casualties in the Meuse-Argonne offensive, returning stateside in May of 1919. It was deactivated later that month.

Other troops were also trained at the camp during World War I. In October of 1917, 190 officers and men from the 1st Separate Negro Company of Maryland arrived at the camp from Pittsburgh. At the outset, the 1st Separate Negro Company was assigned to the Horsed Section of the Ammunition Train, and then later transferred to the Auxiliary Remount Depot near Anniston. Later they were sent to Camp Stuart in Newport News, Virginia, to become a part of the 372nd Infantry. In addition to this group, the 6th Division, 157th Depot Brigade, 11th and 12th Training Battalions, and the 1st, 2nd, and 3rd Development Regiments were all trained at Camp McClellan during World War I.

By February, 1919, 1,534 buildings had been constructed at Camp McClellan. The base hospital alone included 118 buildings. Mess halls dominated as a building type, with showers without heaters and latrines following in that order. Latrines were the most impermanent of the early structures. By 1919, 374 latrines had been replaced with "lavatories". "Kitchen incinerators" were also quickly replaced with more efficient technology.

The year of 1920 witnessed strong debate within the federal government as to the propriety of some of the decisions made as to how military construction should be conducted and under which department's direction. The Corps of Engineers, the Construction Division of the Army, and the Quartermasters Corps were all viable candidates. In June 1920, the Quartermaster Corps was chosen as the most proper supervisory authority. The authors of the history of the Corps of Engineers called the 1920s "the lean years". The future of the military posts of World War I rested in the hands of Secretary of War John W. Weeks. Weeks placed nine of the World War I camps, including Camp McClellan, on caretaker status. The stringency of the cutbacks was appropriate to the mood of the twenties. The war was over, and with it, the need for a standing army. Camp McClellan, like many other military post, fell into disrepair.